Graphene oxide (GO) has a heterogeneous nanostructure, which consists of sp2-graphitic regions and sp3-oxidized regions. In general, the sp2 part of GO has hydrophobic properties, whereas sp3 part represents hydrophilic properties. Also, gas molecules penetrate selectively along the sp3 area, since the gas is absorbed with oxygen functional groups of sp3-oxidized regions. It also act as a binder that controls the interlayer distance between GO nanosheets of layered film structure. However, due to the oxygen functional groups within-plane, high surface charges can cause resistance to reduce the movement of small molecules such as gas and water, which can reduce transport performances. Therefore, the penetration performance of the separator can be adjusted as a ratio of the sp3 parts. It is also commonly known that increased sp2 region reduces the transport of water and gases. In general, sp2/sp3 ratio was adjusted by making reduced GO (rGO) using chemical, thermal, and electrochemical reduction methods but there are disadvantages of remaining chemical additives, structural deformation, difficulty in mass production, and above all, the sp2/sp3 ratio cannot be precisely controlled. Although the mild reduction methods have recently been studied to overcome these problems, there have been no systematic and membrane studies. Here, we report a mild hydrothermal treatment to precisely control the graphitic regions in GO and also tailor its electrical and molecular transport properties. It found that changes in hydrothermal conditions, such as thermal temperature and time, significantly affect the water transport performances as well as gas and salt penetration of treated GO samples. The high-resolution TEM images of hydrothermal treated GO (HGO) samples showed that this mild hydrothermal treatment enables to considerably enhance of sp2-graphitic regions and minimize the formation of structural defects. In addition, the change in electrical conductivity was determined by substituting the percolation theory. Then, we prepared the membrane using hydrothermal treated GOs to measure the gas permeability and selectivity, water permeability, and salt permeability and rejection. Finally, we control the molecular transport properties by controlling electrical performances. This work demonstrates the sustainable performances of GO membranes by redesigning the structure of GO and reveal the transport mechanisms.

Precise control of graphene oxide structure for electrically controllable membranes

2019 Fall National Meeting

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